2017 National Electrical Code Article 240, Overcurrent Protection
Taps supplying a transformer (primary plus secondary not over 7.5m long
(1) the conductors supplying the primary of a transformer have an ampacity at least one third of the rating of the overcurrent device protecting the feeder conductors. (2) The conductors supplied by the secondary of the transformer shall have ampacity that is not less than the value of the primary to secondary voltage ratio multiplied by one third of the rating of the overcurrent device protecting the feeder conductors. (3) The total length of one primary plus one secondary conductor, excluding any portion the primary conductor that is protected at its ampacity, is not over 7.5m (25ft) (4)The primary and secondary conductors are protected from physical damage by being enclosed in an approved raceway or by other approved means. (5) the secondary conductors terminate in a single circuit breaker or set of fuses that limit the load current to not more than the conductor ampacity that is permitted by 310.15.
16 AWG copper.
10 amperes, provided all the following conditions are met. (1) continuous loads not exceed 8 amperes (2) Overcurrent protection is provided by one of the following (a)branch circuit rated circuit breakers listed and marked for use with 16 AWG copper wire (b)Branch circuit rated fuses listed and marked for use with 16 AWG copper wire. (c) Class CC, Class J, or Class T fuses.
Fixture wire shall be permitted to be tapped to the branch circuit conductor of a branch circuit. 40 ampere circuits
12 AWG and larger
Fixture wire shall be permitted to be tapped to the branch circuit conductor of a branch circuit. 50 ampere circuits
12 AWG and larger
Fixture wire shall be permitted to be tapped to the branch circuit conductor of a branch circuit. 20 ampere circuits
14 AWG and larger
Fixture wire shall be permitted to be tapped to the branch circuit conductor of a branch circuit. 30 ampere circuits
14 AWG and larger
12 AWG aluminum and Copper-Clad Aluminum num
15 amperes
14 AWG copper
15 amperes
Fixture wire shall be permitted to be tapped to the branch circuit conductor of a branch circuit. 20 ampere circuits
16 AWG, up to 30m (100 ft) run length
Fixture wire shall be permitted to be tapped to the branch circuit conductor of a branch circuit. 20 ampere circuits
18 AWG, up to 15m (50 ft) run length
12 AWG copper.
20 amperes
10 AWG Aluminum and Copper-Clad Aluminum
25 Amperes
10 AWG Copper
30 amperes
18 AWG Copper
7 amperes, provided all the following conditions are met. (1) continuous loads to not exceed 5.6 amps. (2) Overcurrent protection is provided by one of the following (a)a branch circuit rated circuit breakers listed and marked for use with 18 AWG copper wire. (b) Branch circuit rated fuses listed and marked for use with 18 AWG copper wire (c) Class CC, Class J, or Class T fuses.
Restricted Access Adjustable Trip Circuit Breakers
A circuit breaker that has restricted access the the adjusting means shall be permitted to have an ampere rating that is equal to the adjusted current setting (long time pickup setting) Restricted access shall be defined as located behind one of the following (1)removable and sealable covers over the adjustment means (2)Boled equipment enclosure doors (3)Loced doors accessible only to qualified personnel.
Tap Conductor
A conductor, other than a service conductor, that has overcurrent protection ahead of its point of supply that exceeds the valve permitted for similar conductors that are protected as described elsewhere in 240.4
Current-Limiting Overcurrent Protective Device
A device that, when interrupting currents in its current-limiting range, reduces the current flowing in the faulted circuit to a magnitude substantially less than that obtainable in the same circuit if the device were replaced with a solid conductor having comparable impedance.
Ungrounded Conductors, Overcurrent devices required
A fuse or an over current trip unit of a circuit breaker shall be connected in series with each ungrounded conductor. A combination of a current transformer and overcurrent relay shall be considered equivalent to an overcurrent trip unit. Note, for motor circuits see Parts III, IV, V and XI of article 430.
Overcurrent devices, branch circuit conductors
Brach circuit tap conductors meeting the requirements specified in 210.19 shall be permitted to have overcurrent protection as specified in 210.20
Ungrounded Conductors, Circuit Breaker as overcurrent device.
Circuit breakers shall open all ungrounded conductors of the circuit both manually and automatically unless otherwise permitted in 240.15(B)(1),(B)(3),(B)(4).
Overcurrent devices, feeder taps
Conductors shall be permitted to be tapped without overcurrent protection at the tap, to a feeder as specified in 240.21(B)(1)-(B)(5). the provisions of 240.4(B) shall not be permitted for tap conductors.
Protection of conductors
Conductors, other than flexible cords, flexible cables, and fixture wires, shall be protected against overcurrent in accordance with their ampacities specified in 310.15, Unless otherwise permitted or required in 240..4(A)-(G)
Field Assembled Extension Cord Sets.
Flexible cord used in extension cords made with separately listed and installed components shall be permitted to be supplied by a branch circuit in accordance with the following: 20 ampere circuits-16 AWG and larger.
Ungrounded Conductors, Circuit Breaker as Overcurrent Device, 3 phase and 2 phase systems
For line to line loads in 4 wire, 3 phase system or 5 wire, 2 phase systems, individual single pole circuit breakers rated 120/240 volts AC with identified handle ties shall be permitted as the protection for each ungrounded conductor, if the systems have a grounded neutral point and the voltage to ground Dows not exceed 120 volts.
Supervised industrial installaton
For the purpose of part VIII, the industrial portions of the facility where all of the following conditions are met (1) Conditions of maintenance of engineering supervision ensure that only qualified persons monitor and service the system. (2) The premises wiring system has 2500kVA or greater of load used in industrial process, manufacturing activities, or both, as calculated in accordance with article 220. (3) The premises has at least one service or feeder that is more that 150 volts to ground and more that 300 volts phase to phase. This definition excludes installations in buildings used by the industrial facility for offices, warehouses, garages, machine shops, and recreational facilities that are not an integral part of the industrial plant, substation, or control center.
Fuses or Circuit breakers in parallel
Fuses and circuit breakers shall be permitted to be connected in parallel where they are factory assembled in parallel and listed as a unit. individual fuses, circuit breakers, or combinations thereof shall not otherwise be connected in parallel.
Ground Fault protection of equipment
Ground fault protection of equipment shall be provided in accordance with the provisions of 230.95 for solidly grounded wye electrical systems of more that 150 volts to ground but not exceeding 1000 volts phase to phase for each individual device used as a building or structure main disconnecting means rated 1000 amperes or more. Applying to the disconnecting means for the following (1)continuous industrial processes where a non orderly shutdown will introduce additional or increased hazards. (2) Installations where ground fault protection is provided by other requirements for services or feeders. (3)Fire pumps
Taps not over 3m (10ft) long
If the length of the tap conductors does not exceed 3m (10ft) and the tap conductors comply with all of the following. (1)the ampacity of the tap conductors is; (a) Not less than the combined calculated loads on the circuits supplied by the tap conductors, and (b) Not less than the rating of the equipment containing an overcurrent device supplied by the tap conductors or not less than a rating of the overcurrent protective device at the termination of the tap conductors. (2) The tap conductors do not extend beyond the switchboard, switchgear, panel board, disconnecting means, or control device they supply (3)Except at the point of connection to the feeder, the tap conductors are enclosed in a raceway, which extends from the tap to the enclosure of an enclosed switchboard, switchgear, a panel board, or control devices or to the back of an open switchboard (4) For field installations, if the tap conductors leave the enclosures of vault in which the tap is made, the ampacity of the tap conductors is not less than one tenth of the rating of the overcurrent device protecting the feeder conductors.
Ungrounded Conductors, Circuit Breaker as overcurrent device, Grounded single phase AC circuits
In grounded systems, individual single pole circuit breakers rated 120/240 Volts, with identified handle ties, shall be permitted as the protection for each ungrounded conductor of multiword branch circuits that serve only single phase line to neutral loads.
Ungrounded Conductors, Circuit Breaker as Overcurrent Device, 3 wire DC circuits
Individual single pole circuit breakers rated 120/240 volts DC with identified handle ties shall be permitted as the protection for each ungrounded conductor for line to line connected loads for 3 wire, direct current circuits supplied from a system with a grounded neutral where the voltage to ground does not exceed 125 volts.
Ungrounded Conductors, Circuit Breaker as Overcurrent Device, multiwire branch circuits
Individual single pole circuit breakers, with identified handle ties, shall be permitted as the protection for each ungrounded conductor of multiword branch circuits that serve only single phase line to neutral loads.
Overcurrent protection, grounded conductors
No overcurrent device shall be connected in series with any conductor that is intentionally grounded, unless one of the following conditions are met. (1)The overcurrent device opens all conductors of the circuit, including the grounded conductor and is designed so that no pole can operate independently. (2)Where required by 430.36 or 430.37 for motor overload protection.
Overcurrent devices, Location in circuit
Over current protection shall be provided in each ungrounded circuit conductor and shall be located at the point where the conductors receive their supply except as specified in 240.21(A)-(H). Conductors supplied under these provisions shall not supply another conductor except through an over current protective device meeting the requirements of 240.4
Taps not over 7.5m (25ft) long
Shall comply with the following (1) the ampacity of the tap conductors is not less than 1/3 the rating of the overcurrent device protecting the feeder conductors (2)The tap conductors terminate in a single circuit breaker or a single set of fuses that limit the load to the ampacity of the tap conductors. This device shall be permitted to supply any number of additional overcurrent devices on its load side. (3)The tap conductors are protected from physical damage by being enclosed in an approved raceway or by the other approved means.
Overcurrent Devices rate 800 amperes or Less.
The next higher standard overcurrent device rating (above the ampacity of the conductors being protected) shall be permitted to be used, provide all of the following conditions are met. (1) The conductors being protected are not part of a branch circuit supplying more than one receptacle for cord and plug connected portable loads. (2)The ampacity of the conductors does not correspond with the standard ampere rating of a fuse or a circuit breaker without overload trip adjustments above its rating (but that shall be permitted to have other trip or rating adjustments) (3) The next higher standard rating selected does not exceed 800 amperes.
Adjustable trip circuit breakers
The rating of adjustable trip circuit breakers having external means for adjusting the current setting(long time pick up setting), not meeting the requirements of 240.6(C), shall be maximum setting possible.
240.6 Standard Ampere Ratings. Fuses and Fixed Trip circuit breakers.
The standard ampere ratings for fuses and inverse time circuit breakers shall be considered as shown in table 240.6(A). additional standard ampere ratings for fuses shall be 1,3,6,10, and 601. the use of fuses and inverse time circuit breakers with nonstandard ampere ratings shall be permitted.
Thermal Devices.
Thermal relays and other devices not designed to open short circuits or ground faults shall not be used for the protection of conductors against overcurrent due to short circuit or ground faults, but the use of such devices shall be permitted to protect motor branch circuit conductors from overload if protected in accordance with 430.40
Taps over 7.5m (25ft) long
Where the feeding is in a high bay manufacturing building over 11m (35ft) high at walls and the installation complies with all the following conditions (1)conditions of maintenance and supervision ensures that only qualified persons service the systems. (2)The tap conductors are not over 7.5m long horizontally and not over 30m (100ft) total in length (3) The ampacity of the tap conductors is not less than one third the rating of the overcurrent device protecting the feeder conductors. (4)The tap conductors terminate at a single circuit breaker or a single set of fuses that limit the load to the ampacity of the tap conductors. This single overcurrent device shall be permitted to supply any number of additional overcurrent devices on its load side. (5) The tap conductors are protected from physical damage by being enclosed in an approved raceway or by other approved means. (6) The tap conductors are continuous from end to end and contain no splices. (7)The tap conductors are sized 6 AWG copper or 4 AWG aluminum or larger (8)The tap conductors do not penetrate walls, floors, or ceilings (9) The tap is made no less than 9m from the floor.
Overcurrent devices rated over 800 amperes.
Where the overcurrent device is rated over 800 amperes, the ampacity of the conductors it protects shall be equal to or greater than the rating of the overcurrent device defined in 240.6